Synergistic Activity of Cefiderocol in Combination with Piperacillin-Tazobactam, Fosfomycin, Ampicillin-Sulbactam, Imipenem-Relebactam and Ceftazidime-Avibactam against Carbapenem-Resistant Gram-Negative Bacteria.

Limited treatment options are among the main reasons why antimicrobial resistance has become a leading major public health problem. In particular, carbapenem-resistant Enterobacteriales (CRE), Pseudomonas aeruginosa and Acinetobacter baumannii have been included by the World Health Organization (WHO) among the pathogens for which new therapeutic agents are needed. The combination of antibiotics represents an effective strategy to treat multidrug-resistant (MDR) pathogen infections. In this context, the aim of this study is to evaluate the in vitro activity of cefiderocol (CFD) in combination with different antimicrobial molecules against a collection of well-characterized clinical strains, exhibiting different patterns of antimicrobial susceptibility. Clinical strains were genomically characterized using Illumina iSeq100 platform. Synergy analyses were performed by combining CFD with piperacillin-tazobactam (PIP-TAZ), fosfomycin (FOS), ampicillin-sulbactam (AMP-SULB), ceftazidime-avibactam (CAZ-AVI), meropenem-vaborbactam (MER-VAB) and imipenem-relebactam (IMI-REL). Our results demonstrated the synergistic effect of CFD in combination with FOS and CAZ-AVI against CRE and carbapenem-resistant Acinetobacter baumannii (CR-Ab) clinical strains owing CFD-resistant profile, while the CFD and AMP-SULB combination was effective against CR-Pa strain displaying AMP-SULB-resistant profile. Moreover, the combination of CAZ-AVI/SULB showed synergistic activity in CAZ-AVI-resistant CRE strain. In conclusion, although further analyses are needed to confirm these results, our work showed the efficacy of CFD when used for synergistic formulations.